The potty bell should be loud enough that you can hear it from the next room, and small enough that a pup can easily ring it. A small brass bell on string of sleigh bells would work nicely. Hang the bell from the doorknob of the door to the potty area. The bell should hang at about the height of the pup's nose.
Each time you take your dog to its outdoor elimination area, ring the bell. While you are on your way to the door with your dog say, "Let's go out, go potty." Just before you open the door, ring the bell. You don't have to try to make the dog ring it, just ring it yourself and open the door. The pup will learn to ring the bell itself by imitating you.
Dogs think in a linear way. Whatever happens just before an event, causes the event. If the bell rings and then the door opens, the bell must have caused the door to open. After a few dozen times of hearing the potty bell and then having you open the door, the pup will decide to give it a try.
The puppy will experimentally tap the bell with its nose or paw. Upon learning this sweetest music, you should rush to open the door for your wonderful puppy. The dog notes that the experiment worked and will probably try it again the next time. In anywhere from four days to two weeks, your dog will make that connection between the bell and the door opening, and, from then on, it can ask to go out when he needs to.
Once he figures out how to call you to open the door for him, the pup might try using the potty bell when he wants out for play. Discourage this. Escort your dog out to the potty area when he rings the bell. Don't allow him to play or to wander off. Wait with the pup for a reasonable time and, if he doesn't go, take him back inside. The potty bell is such a valuable communication signal: it must not be used for false alarms.